1. Field of the Invention
The present invention relates to a powder pressing apparatus, a punch, a method for pressing a powder, and a method for manufacturing the punch. More specifically, the present invention relates to a powder pressing apparatus, a punch, a method for pressing a powder, and a method for manufacturing the punch, used in a wet pressing or a dry pressing for manufacturing a compact such as a magnet.
2. Description of the Related Art
According to a powder pressing apparatus of this kind, an upper punch provided with a filter is lowered, then a powder added with water into a form of slurry is loaded in a cavity formed by the upper punch, a lower punch and a die, and then the powder is pressed from above by the upper punch to form a compact while squeezing the water. Then, after the die is lowered, the compact adhering to the lower punch is collected by a vacuum suction.
A contacting surface of the lower punch to which the compact adheres in the above operation is a smooth surface polished by a discharge machining and then finished by lapping.
However, according to the above convention, there are problems if an average grain diameter of the powder is not greater than 1.0 xcexcm, or if the compact is very large. Specifically, in such a case, it becomes difficult to remove the compact from the lower punch, the compact becomes susceptible to fracture by pressing, and a longer time is needed to remove the compact. These problems are caused by a strong adhesion between the compact and the contacting surface of the lower punch finished to a mirror surface by the lapping.
In order to eliminate the cause, two methods were tried. The contacting surface of the lower punch was only discharge-machined in one method, and in the other method the contacting surface of the lower punch was discharge-machined and then ground (coarse polishing operation). However, separability was not improved in either of the methods.
It is therefore an object of the present invention to provide a powder pressing apparatus, a punch, a method for pressing a powder, and a method for manufacturing the punch, capable of improving the separability.
According to an aspect of the present invention, there is provided a powder pressing apparatus for formation of a compact by pressing a powder loaded in a cavity formed by an upper punch, a lower punch and a die, wherein at least either one of the upper punch and the lower punch has a contacting surface for contact with the compact, formed with a groove.
According to another aspect of the present invention, there is provided a punch used in a powder pressing apparatus for formation of a compact by pressing a powder, wherein the punch has a contacting surface for contact with the compact, formed with a groove.
According to still another aspect of the present invention, there is provided a method for manufacturing a punch used in a powder pressing apparatus for formation of a compact, wherein the method comprises a step of forming a groove in a contacting surface of the punch which is to contact the compact.
If it is assumed that the lower punch has the contacting surface formed with the groove, and the compact is removed from the lower punch, according to the present invention, since the contacting surface of the lower punch is formed with the groove, it is presumed that a separating agent is held adequately and uniformly in the entire contacting surface of the lower punch, and that air is easily drawn in between the lower punch and the compact, resultingly preventing adhesion of the compact to the punch at a time of a pressing operation. Further, before the compact is removed from the lower punch, since a pressure acting onto the compact from above is removed by raising the upper punch, it is presumed that a spring back force is released within the compact, developing a gap between the compact and the groove, making the compact off the lower punch. The gap between the compact and the lower punch decreases the adhesion between the two, thereby improving the separability and making easy to remove the compact.
According to the present invention, preferably, the groove is formed at least at an end portion of the contacting surface. It is presumed that this arrangement allows the air to be drawn in more easily between the punch and the compact, further decreasing the adhesion between the two, thereby improving further the separability.
Further, preferably, the contacting surface has a surface roughness Ra=0.05 xcexcmxcx9c25 xcexcm. If the surface roughness is too large, grains of the powder come into the groove, making the compact difficult to separate, whereas if the surface roughness is too small, the adhesion between the punch and the compact is increased to deteriorate the separability. If the surface roughness is set to Ra=0.05 xcexcmxcx9c25 xcexcm, it is presumed that the grains of the powder are prevented from coming into the groove and therefore will not fill the gap between the punch and the compact, and that the separating agent remains adequately within the groove at the time of pressing operation, distributing uniformly in the entire contacting surface, resulting in an improved separability. Since the powder grains do not clog the groove as described above, the punch can be used repeatedly.
Further, preferably, the powder has an average grain diameter not greater than 1 xcexcm. The effect of the present invention becomes significant with the above arrangement. A reason for this is that in general, when the average grain diameter of the powder becomes smaller, a greater adhesiveness is developed between the compact and the contacting surface of he lower punch, deteriorating the separability. However, if he groove is formed in the contacting surface as according to he present invention, it is presumed that the separating agent can be held uniformly in the entirety of the contacting surface even if the average grain diameter of the powder is small, and therefore the separability is improved.
Preferably, the groove is formed at an interval of 0.1 mmxcx9c2.0 mm, and to a depth of 0.2 xcexcmxcx9c100.0 xcexcm. With the above arrangement, the contacting surface of the punch can be formed with the groove consistently and efficiently.
Further, preferably, the groove is formed by milling. With such an arrangement, the contacting surface of the punch can be easily formed with a desired groove. It should be noted here, that a ball end mill or a flat end mill is selectively used depending on a shape of the compact.
According to another aspect of the present invention, there is provided a powder pressing apparatus for formation of a compact by pressing a powder loaded in a cavity formed by an upper punch, a lower punch and a die, wherein the apparatus comprises liquid spraying means for spraying a liquid to the compact formed by pressing the powder and adhered to the upper punch or the lower punch.
According to still another aspect of the present invention, there is provided a method for pressing powder comprising: a first step of forming a compact by pressing a powder loaded in a cavity formed by an upper punch, a lower punch and a die; a second step of spraying a liquid to the compact adhering to the upper punch or the lower punch; and a third step of removing the compact from the upper punch or the lower punch to which the compact adheres, after spraying the liquid.
According to the above invention, particularly when the compact does not separate from the punch, the liquid can be sprayed to the compact adhering to the punch, letting the liquid percolate between the punch and the compact to decrease adhesion between the punch and the compact, improving the separability of the compact, thereby making easy to remove the compact.
According to the present invention, preferably, the sprayed liquid is water. With the above arrangement, it becomes possible to improve the separability of the compact at a low cost, in an environmentally friendly way, without altering characteristics of the compact.
The above-described powder pressing apparatus, the punch, the method for pressing a powder, and the method for manufacturing the punch are effective in wet pressing.
The above object, other objects, features, aspects and advantages of the present invention will become clearer from the following detailed description of embodiments to be presented with reference to the accompanying drawings.